An Inertial Measurement Unit (IMU) can be comprised of multiple inertial sensors; e.g. angular rate sensors (gyroscopes, gyros) and acceleration sensors (accelerometers) arranged to measure the rotational and translational motion of a body. Calculations based on the measured outputs from the inertial sensors can be used to determine the path and/or orientation of a body moving within a physical space. Because of the potential for significant size, weight and cost savings, there is a desire within the art to incorporate MEM (microelectromechanical) inertial sensors into an IMU. However, due to the relatively poor bias stability of currently available MEM inertial sensors (e.g. on the order of 50°/hr, this has not been possible in applications of an IMU that require an accuracy greater than the currently available MEM sensors can provide. The present invention satisfies the need for MEM inertial sensors having improved bias stability, and hence greater sensor accuracy, by providing MEM inertial sensors comprising integral, MEM rotational means. An IMU according to the present invention comprising gyros and accelerometers fabricated in MEM technology with integral rotational means, provides a significant reduction in the cost, size and weight of an IMU, and having an improved accuracy.
MEM fabrication technologies may include surface micromachining, bulk micromachining, focused ion beam (FIB) processing and LIGA (Lithographie, Galvanoformung, Abformung). In the context of this disclosure, micromechanical, microelectromechanical (MEM), integrated microelectromechanical (IMEM) sensors, sense elements, stages, accelerometers, angular rate sensors (e.g. gyroscopes), manufactured using surface micromachining, bulk micromachining, FIB or LIGA technologies, are all included in the definition of a MEM or micromechanical device. Typical dimensions for micromechanical structures comprising a MEM device range from about a micron up to about a millimeter or greater.
An example LIGA technology comprises lithographic processing of a resist material (e.g. polymethylmethacrylate, PMMA) sensitive to x-ray radiation. The PMMA is typically adhered to a flat, planar surface such as that of a silicon wafer, having a thin layer of metal (e.g. as a release layer) on its surface. Highly parallel x-rays from a synchrotron radiation source are incident upon a mask patterned with a high atomic number (Z) absorber (e.g. chromium). The absorbers on the mask are thick enough to prevent the penetration of x-rays. In the open areas of the mask, the radiation passes through and exposes the PMMA resist. The resist is then chemically developed causing the exposed areas to be washed away. The resulting PMMA structure is then used as a mold for an electro-deposition, to produce a metal (e.g. nickel, nickel-iron alloys, copper, gold etc.) part in the developed regions, replicating the outline of the original mask pattern. The metal parts can then be chemically released from the supporting wafer by dissolving away the PMMA and underlying release layer, and used as is, or the electroplated part can be used as a mold for replication from another material such as a plastic or ceramic. The LIGA process produces MEM parts, which must subsequently be assembled to complete a device. Minimum feature size can be approximately 20 microns. Feature thicknesses can range from about 100 micron to 3 millimeter. Feature size to thickness, aspect ratio's, are typically maintained up to about 50:1.
An example surface micromachining technology based on polysilicon structures, adapts planar fabrication process steps found in the integrated circuit (IC) industry, applied to silicon or silicon on insulator (SOI) wafers, to manufacture microelectromechanical or micromechanical devices. The standard building-block processes for polysilicon surface micromachining are deposition and photolithographically patterning of alternate layers of low-stress polycrystalline silicon (e.g. polysilicon) and a sacrificial material (e.g. silicon dioxide or a silicate glass). Vias etched through the sacrificial layers at predetermined locations provide anchor points to a substrate and can form mechanical and electrical interconnections between the polysilicon layers. Functional elements of the device are built up layer by layer by repeating a series of deposition and patterning process steps. After the device structure is completed, it can be released for movement by removing the sacrificial material using a selective etchant such as hydrofluoric acid (HF) which does not substantially attack the polysilicon layers.
The result is a construction system generally consisting of a first layer of polysilicon which provides electrical interconnections and/or a voltage reference plane, and up to three or more additional layers of mechanical polysilicon which can be used to form functional elements ranging from simple cantilevered beams and compliant electrical interconnects, to complex systems such as electrostatic motors connected to a plurality of gears. Typical in-plane lateral dimensions of the functional elements can range from one micron to several hundred microns, while the layer thicknesses are typically about 1 to 2 microns. Because the entire process is based on standard IC fabrication technology, a large number of fully assembled devices can be batch-fabricated on a silicon substrate without any need for piece-part assembly.
An impediment to the application of inertial sensors fabricated in MEM technologies is the poor bias stability of currently available MEM sensors (for example, on the order of 50°/hr). Poor bias stability, also referred to as drift, can lead to errors in the sensors output, thus yielding errors in the calculated position, or calculated orientation, of a moving body to which the MEM inertial sensor is attached.
According to the present invention, a MEM inertial sensor having improved bias stability is provided comprising integral MEM means for rotating the MEM sensor which allows measurement of the sensor's output at different rotational angles. According to the present invention, sum and differencing calculations based on the output of the MEM sensor measured in initial and rotated positions, can determine the bias error of the sensor, allowing the error to be accounted for and/or compensated for in subsequent inertial measurement calculations. The present invention provides significant size, cost and weight reductions in producing an IMU.